GRB 241217a/EP241217B: 1.6m Mephisto optical upper limits

The field of GRB 241217A/EP241217B detected by SVOM/ECLAIRs (GCN 38594) and Einstein Probe (GCN 38606) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 17:09:14 2024/12/17 UT (~12 min after the SVOM/ECLAIRs trigger) and multiple frames with different exposure times were taken. We do not detect any uncatalogued sources in the stacked images within the SVOM/ECLAIRs error region and the 3 sigma upper limits are below. UT start band Exp LimMag (AB) 2024/12/17/T17:09:14 u 300s*3 >21.6 2024/12/17/T17:09:14 g 300s*2, 50s*3 >21.6 2024/12/17/T17:09:14 i 300s*2, 79s*2 >20.6 2024/12/17/T17:13:48 v 300s*3 >21.0 2024/12/17/T17:13:48 r 300s*2, 50s*3 >21.6 2024/12/17/T17:13:48 z 300s*2, 79s*2 >20.3 ------------------------------------------------------------------------------------------------- Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21. -------------------------------------------------------------------------------------------------